Battery charging apparatus

ABSTRACT

The battery charging apparatus is made with small and low-cost components. It includes a first transistor (T 1 ) with a control input through which the charging current flows into a battery (B) and a current source (T 3,  T 4,  R 1,  R 2 ) for a control current flowing to the control input of the first transistor. The current source sets or adjusts the control current so that the first transistor is non-conducting or blocked and the charging current flowing into the battery is shut off when a predetermined maximum charging voltage is reached at the battery.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for charging a battery ora battery charging circuit. This battery charging circuit is especiallysuitable for charging batteries, whose charging can run under voltagecontrol, like, for example, the case of lithium ion and lead batteries.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a miniaturized andinexpensive charging apparatus for charging a rechargeable battery, suchas a lead or lithium ion battery.

The apparatus according to the invention for charging a battery has afirst transistor, through which a charging current passes into thebattery connected to the charging apparatus. A current source isconnected to a control input of this first transistor to supply acontrol current to it. This current source adjusts or sets the controlcurrent for the first transistor so that the first transistor shuts offthe charging current to the battery when a predetermined maximumcharging or supply voltage is reached at the battery.

The apparatus of the present invention is embodied as a chargingcircuit, which is made from very small and also low-cost components.

Further advantageous embodiments are characterized by features claimedin the appended dependent claims.

In preferred embodiments a great simplification of the circuitry resultswhen a Zener diode is connected to the control input of the firsttransistor. As soon as the charging voltage of the battery at the firsttransistor has reached a maximum value, the Zener voltage of the Zenerdiode is exceeded so that the control current no longer flows to thecontrol input of the first transistor, but away through the Zener diode.

In further preferred embodiments it is appropriate to connect a secondtransistor to the first transistor, which keeps the control input of thefirst transistor at a potential, which is less than the charging voltageof the battery connected to the first transistor after shut off of thefirst transistor. Because of that in no case will even a small amount ofcharging current flow into the battery after shut off of the chargingcurrent through the first transistor.

A very high charging current can flow when the charging process isinitiated, which could close or turn on the second transistor and thusinterrupt the charging process. In order to prevent that occurrence thecontrol input of the second transistor is connected to an R/C circuitmeans for delaying the switching on of the second transistor.

The current source works with reduced circuit work because severaldiodes are connected in series in a circuit branch through which thecharging current is conducted and the current source picks off or tapsthe voltage drop across the diodes.

BRIEF DESCRIPTION OF THE DRAWING

The objects, features and advantages of the invention will now beillustrated in more detail with the aid of the following description ofthe preferred embodiments, with reference to the sole figure, which is aschematic circuit diagram of a circuit arrangement for charging arechargeable battery according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The circuit shown in the drawing for charging a battery has two inputterminals 1 and 2, to which an unshown power supply is connectable andtwo output terminals 3 and 4, to which the battery B to be charged isconnected. For example, the battery B to be charged is a lithium ionbattery. The output terminal 3 is connected to the plus terminal of thebattery B and the output terminal 4 is grounded. A plurality of diodesD1, D2 and D3 and a first transistor TI are connected in series betweenthe input terminal 1 and the output terminal 3. The first transistor T1is connected in this series circuit branch so the charging current issupplied through its collector-emitter path to the battery B. The secondinput terminal 2 and the second output terminal 4 are both connected toground.

The series circuit branch including diodes D1, D2 and D3, to which acurrent source is connected, causes a voltage drop in the chargingcurrent path. Also resistors can be used instead of diodes. Generallydiodes have the advantage that the diode voltage is always constant,independent of charging current. This is usually not the case withconventional ohmic resistors. Also resistors heat up with high chargingcurrent.

The current source has two transistors T3 and T4. The base of thetransistor T3 is connected with the collector of the transistor T4 andthe base of the collector T4 is connected with the emitter of thetransistor T3. The emitter of the transistor T4 is connected with thecontrol input (base) of the first transistor T1. The collector of thetransistor T3 is connected with the input terminal 1. Furthermore thebase of the transistor T3 is also connected by means of a resistor R1with the input terminal 1. The emitter of the transistor T2 and the baseof the transistor T4 are connected via a resistor R2 to the controlinput of the transistor T1. Another diode D4 is connected between theinput terminal 1 and the collector of the transistor T3. This diode D4acts as a polarity protector in a known manner. The current sourcecomprising the transistors T3 and T4 and resistors R1 and R2, assubsequently described, adjusts the control current supplied to thecontrol input of the first transistor T1, so that the first transistordisconnects the battery B, when a predetermined maximum charging orsupply voltage is reached at the battery, and thus interrupts thecharging current from input terminal 1 to the output terminal 3.

A Zener diode ZD added between the control input of the first transistorT1 and ground in cooperation with the above-described current sourcecauses the first transistor to become non-conducting when the batteryreaches a predetermined maximum supply voltage and thus the chargingcurrent is interrupted. As long as the battery voltage at the outputterminal 3 is less than the base-emitter voltage UBE1 of the firsttransistor T1, i.e. the Zener voltage of the Zener diode ZD, the controlcurrent supplied by the current source flows nearly completely into thecontrol input (base) of the first transistor and it remains conducting.Only a small portion of the control current flows through a resistor R3connected to ground and parallel to the Zener diode ZD. As soon as thebattery charging voltage at the output terminal 3 has reached thebase-emitter voltage UBEL of the first transistor T1, i.e. a valuecorresponding to the Zener voltage ZD, a large portion of the controlcurrent supplied by the current source by means of the Zener diode ZDflows to ground. That means that the control current for the firsttransistor is so small that it is no longer sufficient to keep the firsttransistor T1 conducting. Then the first transistor T1 opens or becomesnon-conducting and interrupts the charging current to the battery B.

An R/C circuit part, which comprises a resistor R4 and capacitor C,connects a base of a second transistor T2 to the collector of the firsttransistor T1. This guarantees that the first transistor T1 is no longerconducting when a predetermined maximum supply voltage is reached at thebattery B and thus no charging current is supplied to the battery B. Theemitter of the second transistor T2 is connected with the base of thefirst transistor T1 and its collector is connected with the currentsource. If now, as described previously, the maximum supply voltage isreached, the control current at the base of the first transistor T1decreases, so that the voltage drop across the collector and base ofthis first transistor T1 increases. As a result, this voltage switcheson the second transistor T2 and also the transistor T3 of the currentsource. The current source switches off the control current for thefirst transistor T1. Also now the second transistor T2, which forms avoltage divider together with the resistors R1 and R3, determines thevoltage at the base of the first transistor T1. The dimensions of theresistors R1, R3 and the transistor T2 are selected so that the voltageat the base of the first transistor T1 is clearly less than the pluspotential at the output terminal 3 connected to the battery B. Thus thebase-emitter path of the transistor T1 is reliably blocked. No morecurrent can flow into the charged battery B.

The R/C circuit part connected to the base of the second transistor T2causes a delayed switching on of the second transistor T2. Because theturn-on of the second transistor T2 is delayed, the second transistor isblocked and the current source is prevented from being turned off, incases in which a higher charging current should flow when the chargingapparatus is turned on.

When the battery B comprises e.g. a lithium ion cell, a typicalbase-emitter voltage of the first transistor T1 of −5 V suffices for thecharging process. The shut off voltage for the first transistor T1 istypically about −4.1 V. With two or more battery cells connected inseries either a diode (not shown in the drawing) must be connectedbetween the emitter of the first transistor T1 and the battery B, or afirst transistor T1 with a higher permissible base-emitter voltage mustbe used.

It is advantageous to replace the single Zener diode ZD by a pluralityof Zener diodes connected in series each with smaller Zener voltage inother embodiments, which have not been shown in the drawing. These Zenerdiodes have negative temperature coefficients, which compensates for thetemperature behavior of the base-emitter voltage UBEL of firsttransistor T1 and diode D4.

Also in other unshown embodiments a reference voltage for switching offthe first transistor T1 can be produced with other circuit means insteadof the Zener voltage of a Zener diode.

The disclosure in German Patent Application 103 35 018.7 of Jul. 31,2003 is incorporated here by reference. This German Patent Applicationdescribes the invention described hereinabove and claimed in the claimsappended hereinbelow and provides the basis for a claim of priority forthe instant invention under 35 U.S.C. 119.

While the invention has been illustrated and described as embodied in abattery charging apparatus, it is not intended to be limited to thedetails shown, since various modifications and changes may be madewithout departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed is new and is set forth in the following appendedclaims.

1. An apparatus for charging a battery, said apparatus comprising afirst transistor (T1) with a control input, through which a chargingcurrent flows to a battery (B) connected to the first transistor whensaid battery (B) is being charged; and a current source (T3, T4, R1, R2)for a control current for said first transistor, said current sourcebeing connected to the control input of the first transistor (T1) tosupply said control current; wherein said current source comprises meansfor adjusting or setting said control current, so that the firsttransistor is non-conducting or blocked and said charging currentflowing into said battery is shut off when a predetermined maximumcharging voltage at said battery is reached.
 2. The apparatus as definedin claim 1, further comprising a Zener diode (ZD) connected to thecontrol input of the first transistor (T1), said Zener diode having aZener voltage that is exceeded so that the control current no longerflows from said current source (T3, T4, R1, R2) to the control input ofthe first transistor (T1) but flows away through the Zener diode (ZD),as soon as said predetermined maximum charging voltage at said batteryis reached.
 3. The apparatus as defined in claim 1, further comprising asecond transistor (T2) connected to said first transistor (T1), andwherein said second transistor keeps said control input of the firsttransistor (T1) at a potential that is less than the charging voltage ofsaid battery (B) connected to the first transistor (T1) after blockingthe first transistor (T1).
 4. The apparatus as defined in claim 3,further comprising R/C circuit means (R4,C) for delaying a switching onof the second transistor (T2) and wherein said R/C circuit means isconnected to a control input of the second transistor (T2).
 5. Theapparatus according to claim 1, further comprising a series circuitbranch comprising a plurality of diodes (D1, D2, D3) and said firsttransistor (T1) connected in series with each other, and wherein saidcurrent source (T3, T4, R1, R2) is connected across said series circuitbranch so that a voltage drop across said diodes drives said currentsource (T3, T4, R1, R2).